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(1776) its true nature was not known; he showed that oxygen was one of its constituents, but as to its other components he was uncertain. Its exact composition was determined by Cavendish.

Modes of Formation.-(1.) When an electric spark is passed through a detonating mixture of oxygen and hydrogen with which a certain quantity of air or nitrogen is mixed, the water that is produced by the union of the oxygen and hydrogen is found to contain nitric acid. This fact was first observed by Cavendish in the course of his investigations on the composition of water, when, owing to the accidental admixture of air with the mixed gases, oxygen and hydrogen, he found that the water resulting from the union was sometimes acid.

The direct union of nitrogen and oxygen may be brought about by allowing a series of electric sparks to pass between platinum wires in a confined volume of air, contained in a glass globe, as shown in Fig. 49. In a short time the air in the globe will become distinctly reddish in colour, owing to the formation of nitrogen peroxide. The rapidity of the formation of the red fumes will be greatly increased by compressing the air within the globe by means of a small compression pump, as indicated in the figure.

If a small quantity of water be introduced, and the contents of the globe shaken up, the red gas will be seen to dissolve in the water, which will then acquire an acid reaction, owing to the formation of nitric acid.

Similarly, when a jet of hydrogen is allowed to burn in air to which additional oxygen has been added, considerable quantities of nitrogen peroxide are formed. The hydrogen may be burnt .from a jet, surrounded by a glass tube, as shown in Fig. 50, into which oxygen can be passed by means of the small bent tube at the bottom. On holding a clean dry cylinder over the flame, sufficient of the products of combustion will collect in a few seconds to show the presence of nitrogen peroxide.

This direct union of atmospheric oxygen and nitrogen has recently been made the basis of a manufacturing process. A stream of air is caused to pass through the electric arc at a rate sufficiently rapid to sweep away the products of the action and so prevent their dissociation. The nitrogen peroxide which is formed is condensed to the liquid state, and thereby separated from the other gases, by passing the mixture through a refrigerator.

(2.) Nitric acid is formed when nitrogenous animal matter undergoes slow oxidation in the air, in the presence of water and an alkali, the nitric acid combining with the alkali to form a nitrate. In this way nitrates are found in the soil, and from the soil often find their way into shallow well-waters of towns. In hot and rainless countries these nitrates are sometimes found as crystalline deposits on the surface of the soil, as in Chili and India (see Potassium Nitrate).

(3.) Nitric acid is prepared by acting upon potassium nitrate (nitre-saltpetre) with sulphuric acid. The nitre is placed in a glass retort, together with an equal weight of sulphuric acid, and the mixture gently heated. The nitric acid readily distils over, and

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FIG. 49.

Fig. 50.

may be collected in a cooled receiver. The residue in the retort consists of hydrogen potassium sulphate

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The acid so obtained is not entirely free from water, and contains nitrogen peroxide in solution, which imparts to it a yellowish-red colour. To purify it, it is again distilled with an equal volume of sulphuric acid ; and the redistilled acid is deprived of the last traces of dissolved peroxide of nitrogen, by causing a stream of dry air to

bubble through it while slightly warm. Nitric acid so prepared may contain as much as 99.8 per cent. of anhydrous acid, HNO.

(4.) Nitric acid is an article of commercial manufacture. In this process potassium nitrate is replaced by the sodium salt, as being the cheaper material. The proportion of acid to sodium nitrate employed was formerly arranged in accordance with the equation

2NaNO3+H.S0,=Na.S0, +2HNO3. It will be seen that the whole of the hydrogen of the sulphuric acid is thus replaced by the alkali metal derived from two molecules of the nitrate, and that two molecules of nitric acid result. 'This reaction takes place in two stages; in the first we have

(1) NaNO3 + H2S0,=NaHSO4+HNO3. And then, as the temperature is raised, the hydrogen sodium sulphate reacts upon a second molecule of the nitrate, thus

(2) NaNO3 + NaHSO4= Na,SO4 + HNO3. The temperature necessary to effect this second stage, however, causes the decomposition of a certain quantity of the nitric acid

2HNO3=H,O+2NO, +0. And for this and other reasons, most modern manufacturers work only to equation No. 1.

The retorts usually employed for the manufacture of this acid are large cast-iron stills, which are sometimes lined, either entirely or in part, with fireclay, and which are built into a furnace in such a manner as to allow of their being heated as uniformly as possible. The charge of sodium nitrate (12 to 14 cwts.) and sulphuric acid is introduced, and the vapours carried off through an earthenware pipe (c, Fig. 51), connected to a series of earthenware pots, b, in the manner shown in the figure. The last of these jars is connected with a tower, filled with coke, down which water is caused to percolate, and any peroxide of nitrogen which escapes is thereby absorbed. The most modern form of still is not cylindrical, as shown in Fig. 51, but takes the shape of an enormous crucible with a dome-shaped lid ; and is furnished with an exit pipe at the bottom, from which the liquid sodium bisulphate is run off.

Properties.- Nitric acid is a colourless liquid having a specific gravity of 1.53. It fumes strongly in the air, and has a peculiar and choking smell. It is extremely hygroscopic, absorbing moisture

from the air with great readiness. Nitric acid is an intensels corrosive liquid : the strongest acid, when brought in contact et the skin, causes painful wounds, while in more dilute conditions

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it stains the skin and other organic materials a bright yellow colour. A quantity of strong nitric acid thrown upon sawdust causes it to burst into flame. When nitric acid is distilled it first

begins to boil at 86°, at the same time it is partially decomposed into water, nitrogen peroxide, and oxygen ; the distillate, therefore, gradually becomes weaker, and the boiling-point gradually rises. This continues until a certain point is reached, when both the temperature of the boiling liquid and the strength of the distillate remain constant. If, on the other hand, a weak acid be distilled, the distillate gradually increases in strength, until, when the same point is reached, the boiling liquid has again the same temperature.

This constant boiling-point is 120.5', and the distillate which comes over at that temperature contains 68 per cent. of HNO3. Whatever the strength of the acid, therefore, on being boiled it loses either nitric acid or water until the strength reaches 68 per cent., and this liquid boils at 120° C. The specific gravity of this acid at 15° is 1.414. It was formerly supposed that the acid of this strength constituted a definite hydrate, but Roscoe has shown that the strength of the acid is purely a function of the pressure, for by varying the pressure under which the distillation is conducted, acids of various compositions can be caused to distil at a constant temperature. Mixed liquids of this nature are known as constantboiling mixtures, and are strictly analogous to constant-freezing mixtures (page 155).

When nitric acid is mixed with water there is a rise in temperature and a contraction in volume, the maximum effect being produced when the mixture is made in the proportion of three molecules of water with one molecule of acid.

Nitric acid is a powerful oxidising agent, on account of the readiness with which it parts with oxygen. Elements such as sulphur and phosphorus are oxidised into sulphuric and phosphoric acids ; arsenious oxide into arsenic acid ; and many protosalts are converted into persalts. It attacks a large number of metals, forming in many cases the nitrate. Its action upon metals is often of a complicated nature, and depends not only upon the particular metal, but also upon the strength of the acid, the temperature, and the presence of the saline products of the reaction ; thus, when nitric acid acts upon copper, the following reaction takes place

3Cu +8HNO3=3Cu(NO3)2 + 4H,O+2NO.

It is found, however, that as the amount of copper nitrate accumulates, the nitric oxide which is evolved is mixed more and more largely with nitrous oxide, N.,0, and even with nitrogen.

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